Ammonium nitrate explosive



Patented June 6 1961 2,987,389 AMMONIUM NITRATE EXPLOSIVE Wayne A.Proell, Chicago, 111., and William G. Stanley, Hammond, Ind., assignorsto Standard Oil Company, Chicago, 11]., a corporation of Indiana NoDrawing. Filed Feb. 26, 1952, Ser. No. 273,564 7 Claims. (Cl. 52El4)This invention relates to new and improved explosive compositions andalso to compositions for the generation of gas at high pressure.Particularly, the invention relates to explosive compositions whereinammonium nitrate is the principal or sole gas-producing agent. Stillmore particularly, the invention relates to a readily ignitableexplosive comprising ammonium nitrate, an oxidizable material and acombustion catalyst.

Ammonium nitrate is widely used as a component of high explosives.Although ammonium nitrate is classifled as a high explosive, it isextremely insensitive and cannot readily be detonated by the localapplication of heat or by a blasting cap; and when ignited, ammoniumnitrate does not sustain propagation consistently. Normally ammoniumnitrate is mixed with an oxidizable material, such as, sulfur, carbon,cellulosic materials, hydrocarbons, etc., in order to utilize the excessoxygen available in the ammonium nitrate. However, these mixtures ofammonium nitrate and oxidizable materials are also either veryinsensitive or slow burning.

One of the well known methods of overcoming this lack of sensitivity isto use a sensitive high explosive to prime the detonation of theammonium nitrate explosive. Examples of suitable primers are tetryl,TNT, nitrostarch, nitrocellulose, nitroglycerine, etc. An explosive thatis detonable by the action of a blasting cap can be obtained by mixingthe ammonium nitrate with the sensitive materials, such as, nitrostarchand nitrocellulose. The extreme sensitivity of these explosives makesthem undesirable for ordinary blasting use.

Another method of obtaining a readily ignitable ammonium nitrateexplosive is to admix therewith an organic sensitizer, such as, nitrogencompounds and certain carbohydrates. In general, satisfactory explosivesare obtained only when the sensitizer is very intimately dispersedthroughout the mass. Generally this dispersion is obtainable only by theuse of complicated and expensive procedures.

The most commonly used method for improving the sensitivity of ammoniumnitrate explosives is to add a combustion catalyst. The commerciallyused combustion catalysts are all based onthe element chromium. The morecommon chromium combustion catalysts are ammonium or alkali metalchromates or polychromates; chromic oxide, chromic nitrate and copperchromite. The preferred material is ammonium dichromate. While thechromium compounds are the best known combustion catalysts, they havethe disadvantages of being expensive and of, frequently, being in veryshort supply. The relative toxicity of the chromates makes themhazardous unless handled with considerable care. A particulardisadvantage to the use of the chromium combustion catalysts is that theammonium nitrate is sensitized to such an extent that hot meltformulation cannot be used.

This disadvantage has been overcome, in part, by adding a melting pointdepresent to the ammonium nitrate combustion catalyst mixture.

An object of this invention is the preparation of a new and improvedinorganic nitrate explosive, in particular, an ammonium nitrateexplosive. Another object is the preparation of a readily ignitableexplosive comprising ammonium nitrate and a combustion catalyst.

Still another object is to provide a combustion catalyst for ammoniumnitrate explosives which is cheap and in plentiful supply. A particularobject of this invention is an explosive mixture comprising ammoniumnitrate, an oxidizable material and a particular burning catalyst, whichmixture can be formulated by a hot melt process.

Briefly, the explosive mixture of our invention comprises a mixture ofammonium nitrate and an effective amount of a combustion catalystselected from the class of iron-iron cyanide complexes, alkalimetal-iron-iron cyanide complexes and ammonium iron iron cyanidecomplexes. In addition to the ammonium nitrate, moderate amounts ofother inorganic nitrates, such as, potassium nitrate, sodium nitrate andmagnesium nitrate, may be present. We prefer an explosive mixture whichcon tains an oxidizable material in an amount sufficient to utilize theexcess oxygen available from the decomposition of the ammonium nitrate.The amount of combustion catalyst present in the mixture will vary withthe particular catalyst and, somewhat, with the particular oxidizablematerial present. In general, we use between about 0.5 and 25 weightpercent of combustion catalyst.

We have discovered that certain iron compounds are effective catalystsfor sensitizing the ignition and the combustion of ammonium nitrate.When added in sufficient quantity, these catalysts permit ammoniumnitrate or mixtures of ammonium nitrate and. sodium or potassiumnitrate, or mixtures of ammonium nitrate and oxidizable material to beignited readily by the application of localized heat or by a commercialblasting cap. Our combustion catalyst does not appreciably increase theshock sensitivity of the ammonium nitrate and causes only a moderateincrease in the decomposition rate of ammonium nitrate at elevatedtemperatures. Some of the compounds which have been found to beeffective are: ferro ferrocyanide, ferric ferrocyanide, ferroferricyanide, ferric ferricyanide, potassium ferric ferrocyanide, sodiumferric ferrocyanide, ammonium ferric ferrocyanide, potassium-solublePrussian blue, sodiumsoluble Prussian blue and ammmonium-sodium-solublePrussian blue.

All the combustion catalysts of our invention contain the iron cyanideradical, either ferrocyanide or ferricyanide. In addition to the ironcyanide radical, our burning catalyst contains a second iron ion whichmay be either the ferric or ferrous radical. The combustion catalysts ofour invention include those compounds which contain Water of hydrationas well as the anhydrous form. The more effective catalysts contain analkali metal or an ammonium ion in addition to the iron-iron cyanidecomplex. The better catalysts are ammonium ferric ferrocyanide, sodiumferric ferrocyanide, potassium ferric ferrocyanide and mixtures ofsodium or potassium ferric ferrocyanide and ammonium ferricferrocyanide. The extremely complex mixture used as a water soluble bluedye, the so-called soluble Prussian blue, is an excellent combustioncatalyst. The sodium derivative of soluble Prussian blue is anexceptionally effective catalyst and is better than the better knownpotassium derivative of soluble Prussian blue. Some of the commercialsoluble Prussian blues contain both sodium and potassium.

We have found that the most active variety of the combustion catalyst ofour invention is a soluble Prussian blue containing sodium and ammonium.This catalyst must be prepared by a controlled sequence of steps. Thesesteps are: (1) reacting in aqueous solution sodium ferrocyanide andferrous sulfate, (2) precipitation of ferrous ferrocyanide in thepresence of excess ammonium sulfate, and (3) oxidation of ferrousferrocyanide with bromine or sodium chlorate to the ferric ferrocyanide.The product of this procedure contains both sodium and ammonium ions andis spoken free oxygen; additional energy can be obtained by the presenceof an oxidizable material which combines with this free oxygen. Anymaterial which contains a deficiency of oxygen can be utilized. Metalssuch as aluminum and magnesium may be added. The non-metallic elementssulfur and carbon can be used in our explosive mixture.Nitrogen-containing organic compounds that; do not unduly sensitize theexplosive mixture are particularly good; examples of these are urea,nitroguanidine, guanidine nitrate, and mononitrate naphthalene..Cellulosic materials are very desirable oxidizable materials, e.g.,wood flour, cellulose acetate, etc. Because of their cheapness,hydrocarbon materials are a preferred oxidizable material. Examples ofthese are: parafiin -waxes, petrolatum, high boiling hydrocarbon oils,tars, asphalts, bitumen, coal tar, shale oil residue, etc.

The amount of oxidizable material that may be added is dependent uponthe particular type of material, but

' no more oxidizable material should be added than can react with theavailable oxygen to yield soot-free gas. When using hydrocarbonmaterials, in general, the maximum addition is between 20 and 25% byweight based von ammonium nitrate present. We prefer to use explosivemixtures which are stoichiometrically balanced with respect to oxygencontent.

It has been observed that this catalyst burns to a 'fine black dust,much different from the flufiy ash given off by the chromium catalysts.

The stability of ammonium nitrate in the presence of our combustioncatalyst was investigated by measuring the gas evolved from the mixtureat various temperatures. A mixture of 94% ammonium nitrate and 6% ferricferrocyam'de and a mixture of 94% ammonium nitrate and 6%sodium-ammonium soluble Prussian blue was held for eight hours at 135C.; no evolution of gas "was detected. Fresh samples of the abovemixtures were held at 170 C. for several hours; gas was given oif to theextent. of 3 .to 4 ml. per hour per gram of sample. These tests showthat a mixture of ammonium nitrate "and our combustion catalyst issufficiently stable at elevated temperature to permit formulation by ahot melt procedure.

Burning tests were carried out both in air and in an inert atmosphere onmixtures of ammonium nitrate and sodium soluble Prussian blue, ferricferrocyanide, ferric ferricyanide, ferrous ferrocyanide, ferrousferricyanide and sodium-ammonium soluble Prussian blue. The minimumamount of combustion catalyst needed varied with the particularcompound. When using the ammonium-sodium-soluble Prussian blue, amixture of 88% ammonium nitrate and 12% catalyst ignited readily inlarge grains and burned smoothly. When using the iron- Iiron cyanidecomplexes in the absence of the alkali metal ions, it was necessary touse on the order of 15 to 25 weight percent of catalyst in order toobtain easy ignition and smooth combustion.

The effectiveness of our preferred combustion catalyst, i.e.,ammonium-sodium-soluble Prussian blue, was tested on several ammoniumnitrate-oxidizable material mixtures. Test pellets of the desiredcomposition were made as follows: The ingredients were weighed into a100 m1. beaker and then thoroughly mixed by a spatula. In some cases,the mix was ground in a mortar in order to break up small lumps ofammonium nitrate. The mixture was pressed into a pellet about 1%. incheslong and A inch in diameter by means ofa small press using about 50 lbs.pressure on the plunger. In order to insure -trate, 77%; wood flour,15%;

uniformity duplicate pellets were made and tested in each case. Theburning characteristics of each composition were determined afterwrapping each pellet with Scotch cellophane tape so that only the endswere exposed. The wrapping forced the pellet to burn cigarette-fashion.The wrapped pellet was placed on a clay tile and ignited by a Bunsenburner. When the pellet was burning briskly, a 250 ml. beaker was placedover it in order'to determine the effect of inert atmosphere on theburning. Burning rates in inches per second were determined bystopwatch; each rate is an average of at least two trials.

Test 1 The mixture in this test consisted of ammonium niand catalyst,8%. This mixture ignited readily and burned smoothly and completely inan inert atmosphere. The burning rate at atmospheric pressure was 0.0149in./sec.

Test 3 In this test the mixture consisted of ammonium nitrate, 84%;asphalt, 8%; and catalyst, 8%. The mixture ignited readily and burnedsmoothly and completely in an inert atmosphere. The burning rate atatmospheric pressure was 0.0104 in./sec.' The asphalt used in this testhad a flash point of 585 F., a softening point of 98" F. and a densityof 1.010.

Test 4 ture of this. test. The burning rate of the ammonium dichromatemixture was 0.014 in./sec.

Test 5 The mixture in this test consisted of 75% ammonium nitrate, 6% ofcatalyst and 19% of a cellulose acetateglycol diglycolate mix. This testwas carried out in a bomb at 1000 p.s.i. pressure. The burning rate atthis pressure was 0.156 in./sec.

It is obvious that the above mixtures would be effective explosives whenthe combustion occurs in a confined space as is the usual situation foran explosive. The rate of gas generation normally increases withtemperature and pressure in the burning zone and our mixtures can beused to generate gases at high pressure by using a pressure regulator onthe exit conduit.

When using a mixture of ammonium nitrate, a hydrocarbon material and acatalyst containing an alkali metal ion, an ammonium ion or mixturesthereof, we 'prefer to use between about 2 and 10 weight percent ofcatalyst, between about 5 and 25v weight percent of hydrocarbon, and thereminder ammonium nitrate.

The term ammonium nitrate as used in this specification and in theclaims is intended to mean either ordinary commercial grade ammoniumnitrate, such as, conventionally grained ammonium nitrate containing asmall amount of impurities and which is then generally coated with asmall amount of moisture-resisting material such as petrolatum orparaflin, or military grade ammonium nitrate, or a mixture of otherinorganic nitrates and ammonium nitrate wherein the ammonium nitrate isthe preponderate nitrate.

We have. discovered that the effectiveness of our combustion catalystcan be improved remarkably by the addition of certain materials, whichwe call synergists. These materials may or may not be oxidizablematerials and have substantially no catalytic elfect in the absence of acombustion catalyst. The maximum effectiveness is at low concentration,i.e., from about 0.5 to 5 weight percent. When usingthese synergists,the amount of combustion catalyst needed to render the mixture readilyignitable may be as little as 0.5 weight percent. Examples of thesesynergists are: phenothiazine, thiazolidinethione, linseed oil andsucrose; magnesium stearate and magnesium ricinoleate are particularlyeffective with cellulosic materials; aluminum stearate, aluminum oleateand aluminum ricinoleate are particularly suitable when usinghydrocarbons as the oxidizable material.

Our explosive mixture can be made by milling the ingredients or by drymixing; this operation is preferably followed by forming regular shapedcompacted grains by pressing the powdered mix in molds. We prefer toprepare our shaped grains by adding the powdered am monium nitrate tofused organic or oxidizable material at 100-125 C., mixing. to form apaste, and pressing the paste into suitable molds. The cooled grains arestrong and durable.

Having described our invention, what we claim is:

1. A composition for the generation of gas which consists essentially ofammonium nitrate as the predominant component, an oxidizable materialand between about 0.5 and 25 weight percent of soluble Prussian bluecombustion catalyst.

2. The composition of claim 1 wherein said catalyst is prepared byreacting sodium ferrocyanide and ferrous sulfate in aqueous solution,adding thereto excess ammonium sulfate to precipitate ferrousferrocyanide, adding sodium chlorate to oxidize said precipitate andrecovering a soluble Prussian blue containing sodium, ammonium, ferricand ferrocyanide ions.

3. The composition of claim 1 wherein said mixture is aboutstoichiometrically balanced with respect to oxygen.

4. The composition of claim 1 wherein said catalyst contains ammoniumions.

5. The composition of claim 1 wherein contains sodium ions.

6. A readily ignitible explosive mixture which consists essentially ofabout 5 to 25 weight percent of hydrocarbon material, about 2 to 10weight percent of soluble Prussian blue and the remainder ammoniumnitrate.

7. The composition of claim 6 wherein said hydrocarbon material consistsof asphaltic hydrocarbon.

said catalyst References Cited in the file of this patent UNITED STATESPATENTS 43,021 Halvorson June 7, 1864 1,021,882 OBrien Apr. 2, 19121,071,949 OBrien Sept. 2, 1913 1,890,112 Fisher Dec. 6, 1932 FOREIGNPATENTS 14,196 Great Britain 1897

1. A COMPOSITION FOR THE GENERATION OF GAS WHICH CONSISTS ESSENTIALLY OFAMMONIUM NITRATE AS THE PREDOMINANT COMPONENT, AN OXIDIZABLE MATERIALAND BETWEEN ABOUT 0.5 AND 25 WEIGHT PERCENT OF SOLUBLE PRUSSIAN BLUECOMBUSTION CATALYST.